When boilers in power stations require cleaning prior to commissioning, it is common practice to clean the boiler pipes in a purging operation involving discharging high velocity steam through the pipes. The high velocity steam moving through the pipes dislodges or dissolves deposits which are carried away in the form of solid projectiles or liquid drops with the steam. The steam, liquid and projectiles are vented from the boiler pipes to atmosphere at supersonic velocities.
Due to such steam discharges being at high velocity and in large quantities, there is a major problem of avoiding noise pollution. It is known to employ silencers to reduce to acceptable levels the noise of steam venting, but normally such silencers are rapidly destroyed by the combination of extremely high stream velocity and entrained particulate matter. Furthermore in the case of treating steam from geothermal sources which contains large quantities of water, silica and other contaminants, the silencers employed to silence steam emission are partially eroded at the entry by the silica and are contaminated by the silica downstream of the inlet.
Compressed gases or vapours discharged from safety valves may also require silencers. Normally a silencer should have negligible pressure drop to ensure correct resetting of the valve following the cessation of the discharge. Conventional silencers cannot achieve this low pressure drop without resorting either to very large dimensions or introducing flow path modifiers in the form of perforated plate diffusers or multiple wire grids.
It is considered that there is a need to provide new silencers for gas or vapour discharges which can cope with high rates of flow with or without entrained liquids or solids but which are reasonably economic to construct and maintain and are of a reasonable size.